The invention relates to a slave cylinder for a hydraulic actuating system of a motor vehicle friction clutch.
Hydraulic actuating systems of motor vehicle friction clutches must be filled with hydraulic fluid after assembly, in which case it must be ensured that no air bubbles are enclosed in the hydraulic power transmission system between the master cylinder and slave cylinder. Since the subsequent venting of the system is time-consuming, particularly for the series assembly of such hydraulic power transmission systems, it is known to locate two pipes on the reservoir of the hydraulic actuating system by way of a reversing valve, whereof one pipe is connected to a vacuum pump and the other is connected to a pressure pump supplying the hydraulic fluid. By means of the vacuum pump, first of all in one position of the reversing valve, the actuating system is evacuated. Then, by reversing the reversing valve, the system is filled with hydraulic fluid. The filling operation can be carried out quickly in this way, without residual air remaining in the actuating system.
In conventional hydraulic actuating systems, the slave cylinder acts by way of a clutch release fork or the like on a clutch release bearing of the clutch. The slave cylinder of such systems normally has a cylindrical piston, which is sealed with respect to a tubular cylinder by ring packings on its peripheral wall. The ring packings guided for example in peripheral grooves of the piston may receive both the compressive forces occurring during operation, as well as the reduced pressure forces resulting at the time of filling. Since the diameter of the piston is relatively small, even the frictional forces occurring during operation are still acceptable.
The conditions are different if the slave cylinder has an annular cylindrical shape, so that it is arranged concentrically with respect to the transmission input shaft and avoiding a clutch release fork or the like is connected directly to the clutch release bearing of the clutch. Conventional slave cylinders of this type comprise an annular cylinder with an outer cylinder wall, an inner cylinder wall coaxial with the outer cylinder wall and an end wall hermetically connecting the cylinder walls at one end as well as an annular piston arranged to slide axially between the outer and the inner cylinder wall and furthermore a grooved sealing ring able to slide axially together with the annular piston, bearing by radially resilient sealing lips enclosing an annular groove therebetween, both on the outer as well as on the inner cylinder wall, at the end of the annular piston facing the end wall, which together with the cylinder walls and the end wall encloses a hydraulic pressure chamber. A pressure medium connection opens into the hydraulic pressure chamber in a conventional manner. In a slave cylinder of this type, the grooved sealing ring has a comparatively large diameter, with the result that the radial pressure, by which the sealing lips bear against the cylinder walls on account of their inherent elasticity, must be kept comparatively low, if the frictional forces between the sealing ring and annular cylinder are to remain at acceptable values. This problem is all the more serious as the annular piston must be sealed both radially inwards as well as radially outwards. Grooved sealing rings of the aforedescribed type are indeed able to receive compressive forces, but are able to provide a seal with respect to reduced pressure solely to a small extent, since in the case of reduced pressure in the hydraulic pressure chamber, they lift off the cylinder walls, so that in the case of filling assisted by reduced pressure, the actuating system can be evacuated only inadequately.